Present and future automotive fuel specifications point to a significant reduction in sulfur content, mainly in gasoline, with the principal source of organosulfur compounds being fluidized catalytic cracking naphthas (FCC naphthas). FCC naphthas typically have a sulfur content ranging from 1,000 to 1,500 mg/kg. In addition to organosulfurized compounds, FCC naphthas typically have an olefin content from 25-35% by mass.
Conventional hydrodesulfurization (HDS) fixed-bed processes render reduction of the sulfur content in FCC naphtha flows feasible, although the olefins are hydrogenated to some degree, resulting in an unwanted decrease in the octane rating for a gasoline compound containing a hydrodesulfurized flow of an FCC naphtha.
Accordingly, there is a high demand for maintaining the octane rating of gasoline, and thus for sulfur-reduction processes that preserve naphtha olefins. Various processes for the selective hydrodesulfurization of olefinic naphthas are known, wherein selectivity is understood to be reduction of sulfur content while preserving the olefins.
For example, an olefinic naphtha can initially be separated into two distillation fractions so that only the heavy fraction can be subjected to a hydrodesulfurization reaction. Following the reaction, both fractions are restored, and the olefins in the light olefinic fraction can be preserved. This method provides gasoline with a reduced sulfur content while preserving its octane rating. U.S. Pat. Nos. 2,070,295, 3,957,625 and 4,397,739 disclose this type of processing, though with some sulfur remaining in the light naphtha. U.S. patent application 2003/0042175 discloses a process with an additional stage for alkylating thiophenic sulfur in the light naphtha in order to concentrate the sulfur in the heavy naphtha.
U.S. Pat. Nos. 3,957,625, 4,334,982, and 6,126,814 disclose catalytic formulations whose catalyzing characteristics selectively favor hydrodesulfurization while reducing olefin hydrogenation.
Preferably, as the usual hydrorefining catalysts, HDS processes involving olefinic naphthas use catalysts based on transition metal-oxides from Group VI B, preferably MoO3, and transition metal oxides from Group VIII, preferably CoO, in the form of sulfides, supported on an appropriate porous solid. Supports preferably have their acidity reduced by using additives, or else their composition is of intrinsic low acidity. Variations in the metal content are also known, with optimum relationships among them.
U.S. Pat. No. 2,793,170 discloses that low pressures favor a lesser degree of olefin hydrogenation without hindering hydrodesulfurization. The foregoing patent also discloses that, in addition to reactions whereby organosulfur compounds are converted, there is also a reaction recombining the H2S produced by the reactions with the remaining olefins, forming mercaptan compounds. Such reaction makes it difficult to obtain sufficiently low sulfur content in the product without triggering extensive hydrogenation of the olefins. High temperatures also hinder the reaction whereby olefins are recombined with H2S.
The inventor's patent application BR-0202413-6, corresponding to published U.S. application 2004/0000507, discloses using a mixture of at least one added non-reactive compound with hydrogen to trigger selective hydrodesulfurization of a charge of cracked olefin flows. The mixture increases dilution of the hydrogen in the reaction and minimizes olefin hydrogenation without significantly decreasing the conversion of organosulfur compounds. In addition, the mixture decreases the concentration of H2S generated in the reaction and minimizes recombination. It can be seen that a higher ratio of gas per charge volume indicates a decrease in the sulfur content of the product. As to the added non-reactive compounds, it can also be seen that the desired effect of the increased selectivity is achieved not only with nitrogen but also with various diluting compounds and mixtures thereof. It can further be seen that a drop in total pressure does not lead to the same reaction selectivity obtained by using at least one added non-reactive compound. It reduces olefin conversion but increases the sulfur content of the product.
Patent application WO 03/085068 discloses a selected hydrodesulfurization process wherein a mixed charge of naphtha flows with an olefin content of greater than 5% m/m reacts under normal hydrodesulfurization conditions while contacting a selective catalyst. (“m/m” means mass percentage) The process is intended to reduce the sulfur content by more than 90% and to hydrogenate less than 60% of the olefins in the charge. Octane rating loss is expected to be greater from separately treated flows than from naphthas treated as a mixture. Co-processing of a mixture of an olefinic naphtha flow with a non-olefinic naphtha in an amount ranging from 10% to 80% by mass results in at least a 0.1 increase in the octane rating of the final product in comparison to the product processed separately in two charges. Other than a non-olefinic naphtha, no other component is considered for the olefinic naphtha mixture. Further, since naphthas usually have similar distillation temperature ranges, the non-olefinic naphtha will form part of the final gasoline formulation, thereby limiting the application of co-processing.
U.S. Pat. Nos. 6,429,170 and 6,482,314 disclose a process for removing sulfur from catalytic cracking naphtha in a single reaction stage. The process uses a partially sulfided Ni- or Co-based regeneratable reactive adsorbent on a ZnO support. The zinc oxide absorbs the H2S resulting from conversion of the organosulfurized compounds, preventing the recombination reaction, thereby resulting in process selectivity. U.S. patent application 2003/0232723 uses nitrogen in the adsorption process with a regeneratable reactive adsorbent to boost selectivity, wherein the hydrogen molar fraction in the mixture (H2+N2) must be greater than 0.8.
In addition to the single-stage processes described above, and also in order to suppress the recombination reactions, hydrodesulfurization processes have been applied to more than one reaction stage, in which the H2S generated in the reaction is removed between the stages.
U.S. Pat. No. 2,061,845 discloses the use of more than one reaction stage with H2S removed between the stages in the hydrotreatment of cracked gasolines, leading to lesser hydrogenation of olefins and a lower octane rating decrease in comparison to single-stage hydrotreatment. U.S. Pat. No. 3,732,155 discloses the use of two stages with H2S removed between them, and without the charge contacting hydrogen in the second reaction stage.
U.S. Pat. No. 3,349,027 discloses hydrotreatment of olefinic naphthas in two stages, with an intermediate removal of H2S and with a high space velocity (LHSV), making it possible to remove virtually all the mercaptans. Results suggest that the mercaptan reaction rate is rather high, quickly achieving a balance between the olefins present and the H2S in the product.
U.S. Pat. No. 5,906,730 discloses a hydrodesulfurization process for a cracked naphtha in two or more reaction stages, with 60-90% of the sulfur in the charge of each stage removed, allowing for total removal of up to 99% of the sulfur in the original naphtha and with less conversion of olefins in comparison to just one reaction stage.
The H2S generated in each reaction stage is removed before the subsequent stage so as to hinder mercaptan formation resulting from recombination of H2S with the remaining olefins. In U.S. Pat. No. 5,906,730 the operation of the two reaction stages is claimed for specific hydrogen partial pressures between 0.5 to 3.0 MPag in the first stage and 0.5 to 1.5 MPag in the second stage. The claimed hydrogen partial pressure conditions are met under overall pressure conditions and hydrogen flow rates typical for HDS of cracked naphtha. There is no description nor suggestion of any added non-reactive compounds to the hydrodesulfurization reaction aiming at reducing olefin hydrogenation.
U.S. Pat. No. 6,231,753 discloses a hydrodesulfurization process with two reaction stages, with more than 70% of the sulfur removed in the first stage and 80% of the remaining sulfur removed in the second stage, leading to a total removal of more than 95% of the charge sulfur. Between two reaction stages H2S is removed. In order to obtain better selectivity (olefin retention) as compared to previously described two-stage processes, this patent claims a second stage where the temperature and LHSV are higher than those in the first stage: temperature 10° C. higher and LHSV at least 1.5 times higher.
U.S. Pat. No. 6,231,753, reporting the state-of-the-art, teaches that hydrorefining units make use of non-reacted hydrogen for carrying out the reaction and that the consumed hydrogen should be replenished. The same patent further teaches that such hydrogen make-up streams comprise more than 60% by volume hydrogen, and preferably more than 80% by volume, the remaining being inert compounds such as N2, methane and the like.
The so-called inert compounds that may constitute part of the make-up hydrogen result from hydrogen preparation processes. The presence and concentration of so-called inert compounds depend on the presence or not as well as on the efficiency of the H2 purification units. Hydrogen is typically produced in units such as steam reform or as by-product from naphtha catalytic reform. Previously to purification processes, the hydrogen stream from the catalytic reform contains methane and light hydrocarbons, while that from steam reform of natural gas can contain N2. Natural gas used as reform feed can also contain N2 in amounts lower than 10% by volume. Cryogenic processes, membrane separation and molecular sieve adsorption—PSA (Pressure Swing Adsorption) are the most widely used techniques for the purification of such streams. In the technique, inert compounds are considered as undesirable contaminants, so that usually high-purity make-up hydrogen is employed so as to avoid collection of such inert compounds in the gas recycle of hydrorefining units.
U.S. Pat. No. 6,231,753 does not consider the addition of non-reactive or inert compounds as a means for minimizing olefin hydrogenation. On the contrary, said patent teaches that make-up hydrogen is preferably a high-purity stream. In the case the make-up hydrogen stream contains inert compounds, the amount of such compounds in the reaction medium will depend on (i) the recycle flow rate in the system, (ii) the hydrogen consumption, (iii) the make-up flow rate, (iv) the equilibrium in the separator vessels and (v) the presence or not of further treatment of the recycle gas for H2S withdrawal, which can also remove some of the inert compounds.
U.S. patent application 2003/0217951 discloses two reaction stages with H2S removed between them. This process differs from those in the previously cited patents in that more than 90% of the sulfur is converted in the first stage and the reaction rate in the second stage is slower than that in the first stage. A slower reaction rate can be obtained at a temperature lower than that in the first stage.
U.S. Pat. No. 6,736,962 discloses a two-stage process for removing sulfur, with an intermediate H2S removal step between them. A previously hydrodesulfurized olefinic naphtha, containing less than 30 mg/kg of non-mercaptan sulfurized compounds, is processed while contacting a catalyst together with a purge gas, under two possible conditions. When the purge gas is hydrogen, the second-stage catalyst is an irreducible oxide (merely a support, with no hydrogenating activity). When the purge gas is a gas compound, such as He, N2, Ar, CH4, natural gas, light gas, and mixtures of the same containing no hydrogen, the second-stage catalyst is an oxide of a metal from Group VIIIB enhanced by an oxide of a metal from the supported Group VIB (hydrorefining catalyst). The invention does not contemplate mixtures of a purge gas and hydrogen.
Typical conditions for each reaction stage in HDS processes are: pressures ranging from 0.5 to 4.0 MPaG, preferably from 2.0 to 3.0 MPaG; temperatures ranging from 200 to 400° C., preferably from 260 to 340° C.; space velocity (volume processed per hour per volume of catalyst), or SV, from 1 to 10 h−1; rate of hydrogen volume per processed charge volume ranging from 35 to 720 Nm3/m3; and hydrogen purity normally higher than 80%, and preferably higher than 90%. (“Nm3/m3” as a unit for rate of hydrogen volume per processed charge volume means m3 of gas at normal conditions (1 bar, 0° C.) per m3 of feedstock.)
Literature also indicates that when H2S is removed between reaction stages, the H2S concentration at the second stage intake should preferably be less than 0.05% by volume (500 ppmv), or more preferably, the H2S concentration in the gas produced by the second reactor should be less than 0.05% by volume so that it may be recycled back to the first reactor untreated.
Multiple processes are also seen in the art, indicative of the importance and the difficulties inherent to selective processes for removing sulfur from olefinic naphtha flows.
Accordingly, there is still a need for a catalytic hydrodesulfurization process capable of reducing the sulfur content in FCC naphtha charges to the maximum, with minimum hydrogenation of olefins. These objectives have been achieved in the process of the present invention.